This invention relates to an in situ method of retorting oil shale that is particularly effective in the production of liquid and gaseous products from oil shale.
One technique for recovering liquid and gaseous products referred to herein as shale oil, from oil shale is to form an in situ retort in a subterranean oil shale deposit. The oil shale in the subterranean deposit is fragmented to produce the in situ oil shale retort. Typically fragmenting the shale involves forming a void by mining techniques and expanding the adjacent oil shale into the void. This distributes the void volume into interstices between oil shale fragments and provides appreciable gas permeability through the rubble pile of oil shale particles in the in situ report. The fragmented oil shale at the top of the in situ retort is ignited with the aid of a combustible gas and air to form a combustion zone in the in situ retort. After some of the oil shale fragments have been brought to a temperature of about 900.degree. F. air is supplied to the top of the in situ retort to sustain combustion in the combustion zone by the oxidation of the carbon left in the oil shale fragments after the kerogen has been decomposed. Air moving through the combustion zone causes it to proceed slowly through the fragmented shale in the in situ retort. The non-oxidizing fraction of the air, e.g. nitrogen, is heated as it passes through the combustion zone. Such heated nitrogen and the hot products of combustion transfer the heat of combustion to the oil shale in a zone called the "retorting zone" which is on the advancing side of the combustion zone. The kerogen in the oil shale in the retorting zone is converted to shale oil. Thus, a retorting zone moves from the top to the bottom of the in situ retort in advance of the combustion zone, and the resultant shale oil and retort off gases which collectively includes inlet gas, flue gas from the combustion zone and product gas from the retorting zone, pass to a product collection zone adjacent to the bottom of the in situ retort.
When the combustion zone reaches the bottom of the in situ retort, the retort off gases are extremely hot because there is insufficient oil shale between the retorting zone and the point of collection to sufficiently dissipate the heat from the combustion zone. Thus, elaborate arrangements must be installed in the product collection zone to cool the retort off gases to a temperature at which they will not deleteriously affect the product collection and removal apparatus in the product collection zone.
In particular, it is desirable to keep the temperature of the retort off gas and shale oil at the bottom of the in situ retort below about 400.degree. F. The spontaneous ignition temperature of shale oil is about 500.degree. F.; therefore, safety considerations make it preferable to maintain the temperature of the retort off gas moving from the bottom of the in situ retort to the product collection zone at a temperature below about 500.degree. F. Preferably the temperature is below 200.degree. F. to protect seals in product collection and removal apparatus.